I would like to ask if the circulating gas is not desulfurized during the hydrofining and pre-sulfurization process?
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As mentioned, during normal production, our circulating gas needs to be desulfurized by alkali washing. If it is pre-sulfurized, can it not be desulfurized? If desulfurization is not carried out, and the hydrogen sulfide content is so high, wouldn’t pre-sulfurization for dozens of hours pose a corrosive threat to compressors, etc.?
Another question is, how necessary is the initial passivation after pre-sulfide? Some information introduces the use of nitrogen-containing straight-run oil for initial activation and passivation, and some do not. Is it okay if we use the sulfurized oil that has been sulfurized in the reactor as the initial passivation oil? Please give me some experienced advice, thank you!
During the hydrorefining and pre-sulfurization process, the circulating gas is not desulfurized, but the catalyst is changed from the oxidized state to the sulfurized state, which has the strongest activity. The content of hydrogen sulfide is so high that pre-sulfidation for dozens of hours will not pose much of a corrosive threat to compressors, etc.
The hydrorefining and pre-sulfurization process is to change the catalyst from the oxidized state to the sulfurized state. At this time, the catalyst activity is the strongest. Hydrogen sulfide itself does not pose a corrosive threat to the compressor, but care should be taken to control operating conditions!
Could you please explain why hydrogen sulfide itself does not pose a corrosive threat to the compressor? What control conditions should be noted?
When hydrogen sulfide is below 200°C to 250°C, it will cause little or no corrosion to steel. When the temperature is greater than 260°C, the corrosion will accelerate and will intensify as the temperature increases. Especially when the temperature is between 315°C and 480°C, the corrosion rate will increase by 2 times for every 55°C increase. The greater the concentration of H2S and the higher the partial pressure, the more severe the corrosion. The corrosion rate reaches the maximum when the volume concentration of hydrogen sulfide exceeds 1%.
The purpose of pre-sulfidation is to convert the oxidized catalyst into the sulfided state. The molybdenum-nickel catalyst is active only in the sulfided state. Initial passivation is a means to reduce the initial activity in order to prevent the initial activity of the catalyst from being too high, causing violent reactions after switching to fresh materials, and coking on the surface of the catalyst. Whether initial activation and passivation of pre-sulfidation is required depends on the catalyst. Some catalysts have very high initial activity and must be passivated. If the initial activity of the catalyst is not very high, passivation does not need to be performed. You need to ask the catalyst manufacturer. The passivation medium is generally ammonia, and the carrier oil generally uses low-nitrogen oil instead of "nitrogen-containing straight-run oil". This is the same as the requirements for vulcanized carrier oil. After vulcanization, ammonia can be directly injected for passivation.
Our unit currently does not have desulfurization equipment. During vulcanization, the content of hydrogen sulfide in the circulating hydrogen is very good, at least above 5000PPM. I think desulfurization should not be done.
Aren’t you worried about hydrogen sulfide corrosion of the circulating gas compressor?
The oil used for the initial activation and stabilization medium is generally low-sulfur, low-olefin, and low-nitrogen oil.
During the pre-sulfurization process, the vulcanizing agent injection amount is generally 10% higher than the theoretical sulfur injection amount, and the hydrogen sulfide concentration in the circulating hydrogen is kept higher than 1%V to ensure that the metal is fully vulcanized without desulfurization. It is necessary to carry out initial activity passivation of the catalyst in the early stage of operation to prevent the catalyst bed from overheating in the early stage of operation and create conditions for extending the catalyst operation cycle.
Pre-sulfidation of hydrogenation catalysts uses circulating hydrogen to carry H2S to penetrate the catalyst bed to achieve sulfurization, so circulating hydrogen cannot desulfurize. It will not cause corrosion to the circulating hydrogen compressor, because the circulating hydrogen enters the compressor after being cooled to 50°C, and there will be no corrosion problems at low temperatures.
Whether to passivate depends on what kind of raw oil is processed. For example, for catalyst diesel and coke diesel, it needs to be passivated to reduce the initial activity of the catalyst and prevent the temperature from rising too high. Generally, the initial activity of the catalyst is within 200 hours.